This invention relates to a process of treating metal surfaces.
Chromium plating is most widely used for imparting wear resistance and corrosion resistance to iron, steel and alloy steel.
In the method of chromium plating, hexavalent chromium and trivalent chromium contained in the electrolyte react with lead dissolved into the electrolyte from an electrode plate to form lead chromate. Such lead chromate and gas thereof are harmful to human bodies. Although nickel plating, copper plating and other plating methods are also used, use of electrolytes causes public hazard. These plating methods are described in many publications, for example, Metals Handbook, 8th edition, Vol. 2 pp 424-474, published by American Society for Metals.
As a method not accompanying the problem of public hazard, a gas carbonitriding method has been proposed as described in a paper of Tugio Yonemura of the title "Low Temperature Gas Carbonitriding Method", of Denki Seiko, Vol. 46, No. 4, 1975, Nov. pp 227-232. According to this method nitriding is effected in ammonia gas or atmosphere containing ammonia and carbonizing gas and at a temperature higher than 500.degree. C. This method gives more excellent result than hard chromium plating method in weather test and wear resistance test.
Although this method is excellent, use of ammonia accompanies public hazard and the method is expensive.
An ion nitriding method has recently been developed in which the problem of public hazard is not severe and the cost is low.
The ion nitriding method has been well known in the art for a long time and comprises the steps of injecting a mixture consisting essentially of N.sub.2 gas and H.sub.2 gas into a vacuum furnace and applying a DC voltage of several hundreds volts across the furnace acting as the anode electrode and an article to be treated which acts as the cathode electrode thereby establishing glow discharge. As is shown in Takao Talcase's paper of the title "Analysis and Condition of Treatment of Compound Layers" in Kinzoku Zairyo, Vol. 17, No. 5, pp 133-139, such hydrocarbon gas as CH.sub.4 is added to the mixture of N.sub.2 and H.sub.2 for effecting ion nitriding. Articles treated by this method show satisfactory wear resistant property but their corrosion resistant property is not sufficient. For this reason, we have made various experiments in which such carbonaceous gas as CO was incorporated into the mixture of N.sub.2 and H.sub.2. Although, the addition of CO gas has improved the corrosion resistant property good result was not always obtained for any article since respective gas components do not uniformly disperse throughout the furnace so that satisfactory carbonitriding does not occur at nonuniform portions of the mixture. It was also found that the composition of the gas mixture influences the result of treatment. Where the amount of CO gas is excessive it undergoes decomposition to produce carbon which adheres to the article to be treated and prevents nitrization. Consequently the depth of the nitride layer is not uniform.